<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

    <div id="app">
        <div class="box">
            <button @click="buy(5)">牛奶五块</button>
            <button @click="buy(10)">啤酒十块</button>
        </div>
        <hr>
        <div class="shen">余额：{{msg}}</div>
    </div>
    <script src="https://cdn.jsdelivr.net/npm/vue@2.7.16/dist/vue.js"></script>
    <script>
        const app = new Vue({
            el: '#app',
            data: {
                msg: 100
            },
            methods:{
                buy(price){
                    this.msg -= price
                }
            }
        })
    </script>
</body>

</html>